Generally, the conventionally-known vertical, multi-link, adjustable-stroke type vertical engines include: a crankshaft rotatably supported in a crankcase of an engine body; a rotation shaft having its axis line parallel to the crankshaft with an eccentric shaft provided eccentric to the axis line of the rotation shaft; a piston slidably fitted in a cylinder block of the engine body; a main con rod connected at one end to the piston via a piston pin; a sub con rod pivotably connected to a crank pin of the crankshaft and pivotably connected to the other end of the main con rod via a con rod pin; a swing rod pivotably connected at one end to the sub con rod at a position offset from a connected position of the main con rod; and a counter weight section rotatable together with the rotation shaft.
One example of such vertical multi-link, adjustable-stroke type engines is disclosed in Japanese Patent Application Laid-Open Publication No. 2009-275553 (hereinafter referred to as “relevant patent literature”), which can suppress vibration of the eccentric shaft by the provision of the counter weight section rotatable together with the rotation shaft.
In the commonly-known reciprocating type internal combustion engines, combustion pressure applied to the piston is transmitted to a single crankshaft via a con rod and output as rotational force via the single crankshaft (output shaft); thus, normally, the piston combustion pressure acts on only one output shaft.
By contrast, the multi-link, adjustable-stroke type engine disclosed in relevant patent literature includes a plurality of links for controlling the position of the piston. Thus, in a case where a pivot point of one of the plurality of links is connected to the eccentric shaft, combustion pressure applied to the piston is transmitted, via the connected link, to both the crankshaft, which is the output shaft, and the eccentric shaft.
In a case where the eccentric shaft and the crankshaft are interconnected via gears to synchronize rotations of the eccentric shaft and the crankshaft, rotational force caused by combustion pressure acting on the two shafts and by inertia force of motion parts would differ or vary in a very complicated manner depending on an engine load and the number of rotations of the engine. Thus, driving/driven relationship between the gears would also change or switch several times per cycle.
For example, in a case where the crankshaft and the eccentric shaft are interconnected via helical gears, torque switching in response to which driving/driven relationship between the gears changes or switches would cause respective end portions of the crankshaft and eccentric shaft to hit the crankcase etc. each time the gears are changed in operating direction.
Further, in a case where the crankshaft and the eccentric shaft are interconnected via spur gears, no axial load is produced, but the spur gears would have a small contact ratio so that great meshing sound noise occurs.
Furthermore, in a case where the crankshaft and the eccentric shaft are interconnected via double helical gears, both gear meshing sound noise and hitting sound can be reduced, but such double helical gears are more difficult to manufacture and thus tend to be more costly and hence less cost-competitive than helical gears and spur gears.